Lito- i biostratygraficzna granica jury środkowej i górnej na południowym Podlasiu

Abstract

Omówiono osady jury z pogranicza keloweju i oksfordu (w ujęciu L. Malinowskiej, 19670, 1976, 1978a) z kilku nowych otworów wiertniczych obszaru Podlasia. W nawiązaniu do materiałów geologicznych z tych wierceń wrócono do problemu przebiegu granicy między jurą środkową i górną w otworze Radzyń IG 1.

LITHO- AND BIOSTRATIGRAPHIC BOUNDARY OF THE MIDDLE AND UPPER JURASSIC IN SOUTHERN POOLASIE

The sections comprising ammonite-bearing nodular layer are very important for the stratigraphy of Jurassic rocks of eastern Poland as faunal remains are generally rare in that region. The layer makes it possible to delineate accurately the boundary between the Middle and Upper Jurassic in the Lublin-Podlasie area (T. Niemczycka, 1976, a, b). When the nodular layer is present in a section, the boundary is delineated within it as it usually contains faunistic elements d both the Callovian·and Divesian.

In the Lublin and Podlasie regions, the nodular layer occurs at tm: contact of crinoidal and spongy limestones. The age of these limestones is indirectly established with the reference to ammonites recorded in the nodular layer. According to some geologists (K. Dayczak-CaIikowska, 1974), their age is debatable when the nodular layer is not present in a section. According to the present author, the boundary between the Middle and Upper Jurassic always passes at tm: contact of crinoidal and spongy Iimestones in these areas (T. Niemczycka, 1974, a, b) as the nodular layer was nowhere found within crinoidal or spongy Iimestones. There is steadily growing evidence that the layer here always occurs in the same position - at the contact of the crinoidal and spongy limestones (Figs. 2, 3, Tabl. I -4).

The nodular layer yields ammonites of various zones of the Callovian and Divesian but it does not seem to be diachroneous. Its diachroneity is rather apparent, resulting from the scarcity of relevant core material. Most probably it has been formed in the Late Callovian and the earliest Oxfordian in this region. There remains to be solved a very interesting problem of the nature of equivalent deposits originating in places where the nodular layer is missing e.g. in the borehole Radzyń IG 1 area. The geophysical and lithological correlations of the Jurassic sequences from the boreholes Radzyń IG 1, Radzyń IG 6, Łuków IG 2 and Łuków IG 1 (Figs. 2, 3) showed that the equivalents of the nodular layer are represented by top parts of the crinoidal Iimestones and basal spongy Iimestones. It also appeared that the equivalent deposits are relatively thin. Therefore, it may be stated that in the Lublin and PodIasie areas it is justified to delineate the Callovian-Oxfordian boundary at the contact of the crinoidal and spongy limestones when the nodular layer is missing.